For some cultural context:

If I had to sum up Pleroma's core audiences, it's right wing anime posters, Japanese instances that are 2ch-esque and don't really leave their language bubble, privacy and selfhosting folks, terfs, people who like sharing Ukraine war footage and discussing war in general, and pornposters

If I had to sum up Mastodon's core audiences, it's twitter-hating antifascists, European techies, post tumblr fetish sites, lgbt folks, Japanese instances that are 2ch-esque and don't really leave their language bubble, and pornposters

There's a lot of more random folks sprinkled throughout both, but these are definitely the most prevalent communities you'll run into just scrolling the network feeds.